Where is all the antimatter?

Week 22

The 'Alpha Magnetic Spectrometer' – an antimatter detector

Everyone knows about matter; after all, that is what the world is made of (or at least a large part of it). Matter, in simple terms, is what can be touched – that is, physical bodies or objects – in contrast to energy or thoughts and ideas. 'Antimatter', on the other hand, sounds rather spectacular, like the stuff of science fiction. In fact, antimatter is no more extraordinary than 'normal' matter. Matter is made up of minute particles – elementary particles – such as positively charged protons and negatively charged electrons. Antimatter consists of the corresponding antiparticles – that is, negatively charged antiprotons and positrons which – as their name suggests – are positively charged. In antimatter, the electrical charges of the elementary particles are thus precisely the reverse of those in normal matter. If our world were to consist completely of antimatter instead of matter, we would not notice any difference!

However, spectacular things happen when matter and antimatter come into contact: they destroy each other – they are 'annihilated', resulting in pure energy. Conversely, particles and antiparticles can be created from energy. This particle creation only occurs under extreme – high-energy or incredibly hot – conditions. Such conditions are created in modern physics laboratories by machines known as particle accelerators (for instance: DESY in Hamburg or CERN in Geneva).

Particle physics as the key to the secrets of matter

In the early stages of the universe, in the first fractions of a second after the Big Bang, such conditions also prevailed. High-energy radiation was transformed into particles and antiparticles, each in the same quantities. When particle-antiparticle pairs collided, they turned back into radiation and so on – back and forth. This continued until the expansion of the universe led to a fall in temperature and thus the energy of the radiation. As soon as one second after the Big Bang, the temperature had fallen low enough that particles were no longer being created from radiation. But particles and antiparticles could and can continue to be annihilated when they collide.

If it is true, however, that matter and antimatter were created in precisely the same quantities, a problem arises. Why have matter and antimatter not eliminated each other completely? Why is there any matter left at all? Put another way; where is all the antimatter? Today’s universe does not appear to consist of anything but matter. Has there been a spatial separation of matter and antimatter? Are there perhaps far-distant stars and galaxies which consist entirely of antimatter? No evidence of this has been found so far.

Perhaps the 'Alpha Magnetic Spectrometer' (AMS), which can detect antimatter with great precision, can help to answer this question. It is currently being built with German involvement and will be mounted on the International Space Station (ISS) next year.